Describe an example of an endothermic and exothermic reaction. For each of your reactions, identify whether it is a physical or chemical change and explain how the reaction supports the law of conservation of mass.

he sum of the enthalpies of the reactants is greater than the products, the reaction will be exothermic.

Explanation:

Explanation:Endothermic reactions

Photosynthesis

Photosynthesis is the chemical reaction that takes place in green plants, which uses energy from the sun to change carbon dioxide and water into food that the plant needs to survive, and which other organisms (such as humans and other animals) can eat so that they too can survive. The equation for this reaction is:

6

CO

2

(g)

+

6

H

2

O (l)

+

energy

→

C

6

H

12

O

6

(s)

+

6

O

2

(g)

Photosynthesis is an endothermic reaction. Energy in the form of sunlight is absorbed during the reaction.

The thermal decomposition of limestone

In industry, the breakdown of limestone into quicklime and carbon dioxide is very important. Quicklime can be used to make steel from iron and also to neutralise soils that are too acid. However, the limestone must be heated in a kiln (oven) at a temperature of over

900

℃

before the decomposition reaction will take place. The equation for the reaction is shown below:

CaCO

3

(s)

→

CaO (s)

+

CO

2

(g)

Exothermic reactions

Combustion reactions

The burning of fuel is an example of a combustion reaction, and we as humans rely heavily on this process for our energy requirements. The following equations describe the combustion of a hydrocarbon such as petrol

(

C

8

H

18

)

:

fuel + oxygen

→

heat + water + carbon dioxide

2

C

8

H

18

(l)

+

25

O

2

(g)

→

16

CO

2

(g)

+

18

H

2

O (g)

+

heat

This is why we burn fuels (such as paraffin, coal, propane and butane) for energy, because the chemical changes that take place during the reaction release huge amounts of energy, which we then use for things like power and electricity. You should also note that carbon dioxide is produced during this reaction. The chemical reaction that takes place when fuels burn has both positive and negative consequences. Although we benefit from heat, power and electricity the carbon dioxide that is produced has a negative impact on the environment.

Respiration

Respiration is the chemical reaction that happens in our bodies to produce energy for our cells. The equation below describes what happens during this reaction:

C

6

H

12

O

6

(s)

+

6

O

2

(g)

→

6

CO

2

(g)

+

6

H

2

O

(l)

+

energy

In the reaction above, glucose (a type of carbohydrate in the food we eat) reacts with oxygen from the air that we breathe in, to form carbon dioxide (which we breathe out), water and energy. The energy that is produced allows the cell to carry out its functions efficiently. Can you see now why you must eat food to get energy? It is not the food itself that provides you with energy, but the exothermic reaction that takes place when compounds within the food react with the oxygen you have breathed in!

Fact:

Lightsticks or glowsticks are used by divers, campers, and for decoration and fun. A lightstick is a plastic tube with a glass vial inside it. To activate a lightstick, you bend the plastic stick, which breaks the glass vial. This allows the chemicals that are inside the glass to mix with the chemicals in the plastic tube. These two chemicals react and release energy. Another part of a lightstick is a fluorescent dye which changes this energy into light, causing the lightstick to glow! This is known as phosphorescence or chemiluminescence.

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